Fix constraint file open error message

[gcomneno]

Fixes #13226.

When a constraint file path is missing/unreadable (via -c/--constraint or PIP_CONSTRAINT), pip reported "Could not open requirements file", which is misleading.

This change passes the existing constraint context into get_file_content() and tailors the InstallationError message to say "constraint file" when appropriate.

Tests: add a unit test asserting the correct message for constraint=True.

[gcomneno]

Thanks! FYI, earlier CI noise was due to aligning the Towncrier fragment naming with Chronographer and whitelisting news/** as git-only for check-sdist (fragments are dev-only, not included in sdist). Latest head is green.

[pfmoore]

What is the point of this change? It seems unrelated to the purpose of the PR.

[gcomneno]

Good point - it isn’t directly related to the UX fix. I added it while addressing CI failures around the Towncrier fragment, but I agree it’s scope creep for this PR. I’ve reverted the check-sdist change; if the project wants an explicit adjustment for Towncrier fragments, I can follow up with a dedicated PR.

[pfmoore]

Your NEWS entry is named incorrectly. See here:

If your issue or PR number is 1234 and this change is fixing a bug, then you would create a file news/1234.bugfix.rst.

Please fix the filename and rebase the PR to remove the spurious commits messing with the news file and whitelisting.

[gcomneno]

Thanks for the review! I've renamed the news fragment to news/13226.bugfix.rst and rebased to drop the intermediate/spurious commits. Latest head is green.

[pfmoore]

Thanks for the fix for this small, but annoying, issue! 🙂